1. Field of the Invention
This invention relates to wireless communications, and more specifically to correction of shifts that occur due to Doppler effects imposed on transmitted signals.
2. Background Information
Many applications for wireless communications include obtaining timing and/or positional information from a received signal. Such applications may include locating and tracking a particular feature or pattern in a received signal over time. In one example, a base station tracks a signal received from a mobile telephone to determine the telephone's relative velocity. In another example, a receiver calculates its position by identifying delays in signals received from satellites in a global positioning satellite (GPS) system.
In a direct sequence spread spectrum (DSS) technique, a data signal is spread with a pseudo-noise (PN) code sequence before being modulated onto a carrier. In a GPS system, one such code sequence is called a coarse acquisition (C/A) code. When the received signal is correlated with the same code sequence, a correlation peak is obtained. The location in time of the correlation peak provides information regarding the propagation delay of the signal. This propagation delay information may be used to determine the relative distance between the transmitter and the receiver.
It may be desired to integrate correlation results over time, whether coherently and/or noncoherently. For example, an individual correlation result may be too weak to distinguish from background noise. Accordingly, integration over time may be employed to improve the signal-to-noise ratio (SNR).
Relative motion between a transmitter and a receiver causes a Doppler shift in the received signal. One effect of a Doppler shift is to cause the spreading code sequence in the received signal to drift over time. When correlating a Doppler-shifted signal over time, the correlation peak becomes distorted. The distortion typically appears as a spreading of the correlation peak as compared to the correlation peak resulting from correlation of a non-shifted copy of the signal. Such distortion may reduce the signal-to-noise ratio (SNR) of the correlation peak. This can be understood by recognizing that the amount of energy in the peak is the same whether distorted or not. However, in the distorted peak, the amount of energy is more spread out. In addition, this distortion or spreading of the peak may reduce the accuracy to which the location in time of the correlation peak may be determined.